1. Field of the invention
The field of the invention is that of signal transmission systems, in particular systems transmitting digital signals modulated by a form of modulation such as Quadrature Phase Shift Keying (QPSK) in which the signals are transmitted over a multipath dispersive medium.
The invention is more particularly concerned with a receiver system for processing signals transmitted over a multipath dispersive medium and received on a plurality of diversity channels.
2. Description of the prior art
It is standard practice to use adaptive digital equalizers in receiver systems to minimize the disturbing effects of the transmission medium. In particular, multipath propagation of signals generates inter-symbol interference. This is the case with transmission by a line of sight or tropospheric microwave beam, for example. The signal received by the receiver system is then the time-varying weighted sum of differently delayed different replicas of the transmitted signal.
Digital equalizers are usually implemented in the form of digital tranversal filters with multiple coefficients and corresponding symbol taps. Adaptive digital equalizers may incorporate a backward decision filter which offers an improvement in performance, especially on media subject to severe distortion.
The performance of an adaptive digital equalizer is critically dependent on the phase of the symbol-clock signal regenerated locally in the receiver to digitize the demodulated received signals and to clock the digital equalizer at the same rate as the digitized complex samples.
Known adaptive digital equalizers include oversampling adaptive digital equalizers in which the demodulated received signal is sampled at a frequency which is a multiple of the symbol-clock signal frequency. Oversampling adaptive digital equalizers comprise digital filters adapted to vary their group propagation time according to the phase of the symbol-clock signal using algorithms such as the stochastic gradient algorithm, for example, to calculate coefficients of the digital transversal filters which automatically compensate phase errors of the symbol-clock signal.
In practice, however, it is found that this advantageous feature of oversampling adaptive digital equalizers is not sufficient to compensate for serious phase errors in the regenerated symbol-clock signal because the number of coefficients of the digital filters is necessarily limited. Consequently it is necessary to provide a symbol-clock signal control device to monitor and correct the phase of the symbol-clock signal to ensure correct operation of the digital equalizer.
The document IEEE TRANSACTIONS ON COMMUNICATION, vol.COM-24, N.degree. 8, AUGUST 1976, pages 856-863 discloses a receiver system comprising an oversampling adaptive digital equalizer and a timing recovery device controlling means for generating a symbol-clock signal. The timing recovery device derives a phase control signal from the coefficients of the transversal filter of the adaptive digital equalizer to control the phase of the generated symbol-clock signal. The control signal is representative of the weighted sum of the coefficients of the equalizer filter. This weighted sum is expressed by the equation: ##EQU1## in which M has a selected value slightly greater than N/2. Thus movement of the coefficients of the filter causes the value D to vary either side of the null value. The value D is filtered to form the phase control signal which controls the phase of the symbol-clock signal.
However, it has been found that the performance of the clock recovery device remains mediocre because the range of adjustment, in terms of bandwidth, of the phase of the symbol-clock signal is essentially limited by the loop bandwidth of the digital equalizer, which is a few hertz.
What is more, the performance of the equalizer is degraded because the phase control signal is generated from the movement of the coefficients of the equalizer filter, which must adapt to random variations in the phase of the symbol-clock signal, whereas the transmission medium may remain stable. This leads to the selection of costly highly stable clocks.
Finally, this timing recovery device is not suitable for a digital equalizer with multiple diversity channels comprising multiple digital transversal filters. The above equation caters for only one diversity channel and it would not seem possible to expand this equation to cover multiple diversity channels.
An object of the invention is therefore to alleviate the aforementioned drawbacks and in particular to solve the problem of symbol-timing recovery in a receiver system comprising an adaptive digital equalizer with multiple diversity channels.